FRIDAY, Oct. 13, 2017 (HealthDay News) -- The active ingredient in "magic mushrooms" may help patients with tough-to-treat depression, a new study suggests.

Twenty patients received psilocybin -- the psychoactive compound in a group of mushrooms that cause hallucinations. Nineteen who completed the study showed improvement in their depression symptoms for up to five weeks after treatment, according to the researchers at Imperial College London.

"We have shown for the first time clear changes in brain activity in depressed people treated with psilocybin after failing to respond to conventional treatments," said study leader Robin Carhart-Harris, head of psychedelic research at Imperial.

However, his team emphasized that patients with depression should not try to self-medicate with magic mushrooms. While these results are promising, the study was small and did not include a comparison group of patients who did not receive psilocybin, they noted.

Still, brain scans before and after treatment suggest psilocybin may reset the activity of brain circuits that play a role in depression.

"Several of our patients described feeling 'reset' after the treatment and often used computer analogies," Carhart-Harris reported in a college news release. One said he felt like his brain had been "defragged" like a computer hard drive, and another said he felt "rebooted," the researcher added.

"Psilocybin may be giving these individuals the temporary 'kick-start' they need to break out of their depressive states, and these imaging results do tentatively support a 'reset' analogy. Similar brain effects to these have been seen with electroconvulsive therapy," Carhart-Harris said.

Larger studies are needed to see if this positive effect can be reproduced in more patients, said study senior author David Nutt.

"But these initial findings are exciting and provide another treatment avenue to explore," said Nutt, director of neuropsychopharmacology in the division of brain sciences.

He added that a trial scheduled to start early in 2018 will test the psychedelic drug against a leading antidepressant.

For this latest study, patients received two different doses of psilocybin, one week apart.

In recent years, promising results have emerged from a number of clinical trials testing the safety and effectiveness of psychedelics in patients with conditions such as depression and addiction.

"Psilocybin can be a promising agent for depression," said Dr. Scott Krakower, assistant unit chief of psychiatry at Zucker Hillside Hospital, in Glen Oaks, N.Y. "For decades, there has been suspected benefit of psychedelic agents for the treatment of psychiatric and neurological disorders."

However, he said, the clinical trials to date have been very small, and without a placebo arm for comparison. He agreed that replication in larger studies is warranted.